The present invention provides incandescent lampshaving a tungsten filament with a glass lamp bulb, preferably hard glass, with the lead wires hermetically embedded and thereby sealed in the glass. The lamp has an inert gas fill including a halogen additive, e.g., bromine or HBr, and functions with a regenerative cycle. During operation the partial pressure of the halogen functions so that the tungsten particles evaporated off the filament are captured so that they do not reach the glass of the lamp bulb. Instead, they are returned to the filament. Incandescent lamps with a regenerative cycle remain unblackened throughout their service life. As a consequence, their luminous efficacy remain substantially constant during their service life. Lamps of this type are known, for example, see U.S. Pat. No. 3,418,512.
During the manufacture of such incandescent lamps, a specified quantity of a brominated hydrocarbon, such as dibromomethane, is added to the inert gas fill. The brominated hydrocarbon is subsequently thermally decomposed by heat within the lamp bulb, upon initial lighting of the lamp. The decomposition may even occur earlier while passing the gaseous mixture through the exhaust tube when the lamp is being filled. In addition to the brominated hydrocarbon, a specified quantity of carbon oxide may be added to the inert gas fill. A quantity of from 0.05 to 0.5 volume percent of carbon oxide is feasible, see DT-OS No. 2,046,186.
Hard glass useful in the manufacture of the lamp bulb of such incandescent lamps is resistant to high temperature, see, for example, the German published application No. P 26 01 576.9 filed on Jan. 16, 1976 which corresponds to U.S. application Ser. No. 755,189 filed on Dec. 29, 1976, now abandoned in favor of Ser. No. 869,494, filed Jan. 16, 1978, which discloses borosilicate hard glass. Such hard glass is resistant to operating temperatures of above 700.degree. C. They usually contain sodium among other components. The glass has dissolved therein carbon oxide, carbon dioxide, and water. Such materials may be introduced into the glass melt, for example, as moist air when preparing the powdered glass batch constituents; in the form of gaseous decomposition products, for example, steam and carbon dioxide, of the glass batch constituents which are not added as the oxide but as the hydrate, hydrated salt, carbonate, etc.; and lastly gases which diffuse into the fused glass, such as for example, adsorption of combustion or combustible gases. During operation of the lamp such materials diffuse out of the glass into the lamp bulb and may adversely affect the regenerative cycle, thereby reducing the service life of the lamp. Carbon monoxide, carbon dioxide, and water which speed up the regenerative cycle, and/or hydrogen and sodium and potassium which slow down the regenerative cycle, may be released during the operation of the lamp and the composition of the fill gas may be subject to an unwanted change.
The materials which speed up the regenerative cycle cause noticeable transport of tungsten and molybdenum which can be distinguished from surface roughness and dendrite growth on the internal lamp components. When the quantity of substances which speed up the regenerative cycle is small, then it is possible to compensate for the negative influence on the lamp's service life by a selection of the amount of the halogen additive such that it minimizes the said negative affect. The substances which slow down the regenerative cycle have a getter effect on the halogen additive within the lamp. Thus, when an alkali, for example, sodium, is present, stable sodium halogen compounds are formed which precipitate. With a sufficiently large amount of sodium present, this finally leads to a sufficient deficiency in the amount of halogen available so that the regenerative cycle breaks down with consequent blackening of the lamp. Although it would be possible to postpone the breakdown of the regenerative halogen cycle by using an excess amount of halogen in the lamp, the result of the excess amount of halogen present would be excessive functioning of the regenerative cycle. As a consequence, the more it is attempted to postpone the onset of lamp blackening in this manner, the sooner will the lamp become unserviceable as a consequence of the excess rate of the regenerative cycle.
It is an object of the present invention to prevent the reduction in service life of such lamps by minimizing or preventing the inclusion in the hard glass of the lamp bulb of substances which speed up or slow down the regenerative cycle and which might diffuse into the interior of the lamp during operation.